1. Field of the Invention
The present invention relates to a grounded emitter amplifier and a radio communication device using the same. Specifically, the present invention relates to a grounded emitter amplifier suitable for application in an integrated circuit and a radio communication device using such grounded emitter amplifier.
2. Description of the Related Art
A typical circuit configuration for a conventional grounded emitter amplifier is exemplified in FIG. 4, in which an emitter of a transistor 101 is connected to the ground through an emitter resistor 102, and a collector of the transistor 101 is connected to an electric power source supplying a voltage VCC through a load resistor 103. In addition, a base of the transistor 101 is biased by a constant bias voltage Vbias supplied by a direct-current voltage source 104 through a base resistor 105. Then, by applying an input signal Vi to a terminal Vin connected to the base of the transistor 101 through a capacitor 106, an output signal Vo is led out from a terminal Vout connected to a collector of the same transistor 101.
As the constant bias voltage Vbias being applied to the base of the transistor 101, the grounded emitter amplifier like described above has the problem of changing an emitter current Ie of the transistor 101 when variations in several parameters of the transistor such as a current amplification factor hfe occur. This problem is explained through the mathematical expressions shown below:
when a base current of the transistor 101 is Ib and a collector current of the same transistor 101 is Ic, an emitter current Ie is obtained from
Ie=Ib+Icxe2x80x83xe2x80x83(101); 
in addition, the collector current Ic is obtained from
Ic=hfexc3x97Ibxe2x80x83xe2x80x83(102); 
then yields, from equations (101) and (102),
Ie=(1+hfe)xc3x97Ibxe2x80x83xe2x80x83(103). 
Therefore, the emitter current Ie of the transistor 101 is changed according to the current amplification factor hfe of the same transistor 101. When the emitter current Ie is changed, variations in power consumption and variations in a resistance value of the emitter resistor 102 of the transistor 101 itself occur, thus causing variation in gain.
It is an object of the present invention to provide a grounded emitter amplifier and a radio communication device using the same. The amplifier of the present invention has a construction in which an emitter current of a transistor is not influenced by variations in several parameters of the transistor such as a current amplification factor hfe.
The amplifier according to the present invention has a configuration comprising an amplification circuit having a grounded emitter first transistor and a base first resistor connected between a base electrode of the first transistor and a bias terminal; a grounded emitter second transistor having a collector electrode connected to the bias terminal; an operational amplifier including an inverted input connected to an emitter potential of the second transistor; a non-inverted input connected to a predetermined direct current voltage; and an output terminal connected to the bias terminal mentioned above; and a bias voltage generation circuit having a base second resistor connected between the output terminal of the operational amplifier above and a base electrode of the second transistor.
The amplification device of the present invention as described above satisfies the condition in which, when a resistance value of the base resistor of the first transistor is Rb1 and, likewise, a resistance value of the base resistor of the second transistor is Rb2, and an emitter area of the same first transistor is Q1 and, likewise, an emitter area of the same second transistor is Q2, then
Rb1=Rb2/n 
and
Q1=nxc3x97Q2, 
where n is a positive number.
The factor n such as described in the equations above can be realized easily by applying a monolithic IC technology.
In addition, the amplification device of the present invention realized according to the construction mentioned above can be used to constitute a means of amplification of an intermediate-frequency (IF) signal for a radio communication device.
According to the amplification device and the radio communication device of the construction mentioned above, the grounded emitter amplification circuit and the bias voltage generation circuit satisfying the conditions described above allow the bias voltage generation circuit to generate and supply to a grounded emitter amplification circuit a bias voltage that adjusts the emitter current of a transistor in the grounded emitter amplification circuit so that such emitter current does not receive an influence of variations in several parameters of the transistor such as a current amplification factor hfe.
In other words, if a grounded emitter amplifier and a radio communication device using the same are constructed according to the present invention, then the grounded emitter amplifier and the radio communication device using the same have the characteristics of generating a bias voltage that adjusts the emitter current of the grounded emitter amplification circuit without receiving an influence of variations in several parameters of a transistor of the grounded emitter amplification circuit such as an electric current amplification factor hfe. By supplying such a bias voltage to the grounded emitter amplification circuit, it is possible to obtain an emitter current that does not receive an influence of variations in several parameters of the transistor such as the electric current amplification factor hfe.